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  1. #1

    Effects of Oyster shells

    how does oyster shells affect pH?

  2. #2
    If the water is slightly acidic it is supposed to eat into the shell which is high in calcium that raises your ph

  3. #3
    An understanding of pH reveals that itís bicarbonate that buffers the value of pH. It raises the KH (carbonate hardness) and it's the interplay between bicarbonate converting to and from carbonate that stabilises or buffers pH. You can buy dirt cheap sodium bicarbonate that it will work well with a predictable effect in any pond.

    The subject of ďis calcium carbonate any good as a buffer?Ē is a difficult concept to understand so I'll explain it as slowly as possible in easy steps.

    • There are many forms of calcium carbonate that you can put into a pond such as limestone chips or powder, oyster shells, eggshells, calcified algae etc.
    • Calcium carbonate is poorly soluble in water unless carbon dioxide is also present. If carbon dioxide is present then the calcium carbonate reacts with it to make calcium and bicarbonate both of which are soluble in water.
    • If you put any form of calcium carbonate into a koi pond and the pond is very well aerated then there won't be a significant level of carbon dioxide. Without carbon dioxide being present the calcium carbonate won't dissolve to any great extent which means that very little bicarbonate will be produced so it will do very little to stabilise the pH.
    • If you put any form of calcium carbonate into a pond that isn't well aerated it will dissolve according to how much carbon dioxide is dissolved so some bicarbonate will be produced and that will have some effect on the stability of the pH.
    • It all depends on how poorly aerated the pond is. If it isn't too bad and there's only a small amount of carbon dioxide present there will be a small buffering effect. If the aeration is very poor and there's a lot of carbon dioxide present there will be a lot of bicarbonate produced so there will be a larger buffering effect.
    • It isnít true to say that limestone or shells won't dissolve in a koi pond. What is true is that calcium carbonate is virtually insoluble in water unless there also is dissolved carbon dioxide present. This is the important statement to remember.
    • If carbon dioxide (CO2) is dissolved in water it forms carbonic acid (H2CO3) and it's the amount of dissolved CO2 that makes the effect of shells unpredictable. If there is a high level of dissolved CO2 then shells will dissolve freely. If there is good aeration which, not only gives high dissolved O2, but also near-zero dissolved CO2 then calcium carbonate (CaCO3) will be nearly insoluble and therefore shells and other forms of calcium carbonate will have no significant effect on pH swings.
    • I can do the chemistry if anyone wants me to show what actually happens but the bottom line is that, if anyone finds that shells dissolve in their pond then they either have allowed their pH to fall below 7.0 or they have a significant level of dissolved CO2.
    • A final point is that sodium bicarbonate will attempt to buffer the pH to a maximum value of 8.4 and the rise in KH is predictable because it can easily be calculated.
    • The actual pH if calcium carbonate is used as a buffer pH isn't predictable because it depends on the level of dissolved carbon dioxide that is present but its maximum pH value is in the region of 10 to 11.

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  5. #4
    Junior Member Rank = Fry JasonOB's Avatar
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    Apr 2021
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    if the pond water is acidic enough to eat in to the shell, the koi are probably long dead...

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  7. #5
    thinking along the lines of marine calcium reactors... could the dissolving of the shell also release phosphates into the water in potentially problematic amounts?

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  9. #6
    Quote Originally Posted by clippo View Post
    thinking along the lines of marine calcium reactors... could the dissolving of the shell also release phosphates into the water in potentially problematic amounts?
    Oyster shells are almost all calcium carbonate. There will be trace amounts of other minerals and compounds which will include some phosphate but only a very small amount so there won't be a significant release of phosphate when they dissolve.

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  11. #7
    Senior Member Rank = Jussai davethefish1's Avatar
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    Quote Originally Posted by clippo View Post
    thinking along the lines of marine calcium reactors... could the dissolving of the shell also release phosphates into the water in potentially problematic amounts?
    having run a reef tank with a calcium reactor i did wonder if one would be workable on a koi pond...
    especially if running RO or having extremely low KH mains water.

    but the amount of water changes done on a pond scale, would mean you would need an big reactor, chewing through loads of Co2 bottles.
    it might be workable on an anoxic pond with little water changes and a need for mineral addition.
    but it's just easier and cheaper to add sodium bicarbonate...

    i am thinking of digging out my old deltec AP851 skimmer out of the loft to run on the pond though, just to see if it does much of anything
    The Solution to Pollution is Dilution

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  13. #8
    Firstly, since an anoxic pond is a part of the biological filtration, the full turnover rate of the main pond flows through it as with any other form of biological filtration so there is no difference between the water chemistry in the main pond and an anoxic pond.

    The CO2 injection types of pH controllers add CO2 to the water in order to lower the pH because dissolved CO2 is acidic. If you try to lower the pH in a koi pond by injecting CO2, the koi will experience the same effect as you would in a stuffy room where there is a build up of CO2 due to inadequate ventilation. Even so, the injected CO2 would quickly be gassed off by aeration so the pH would always be trying to go back up again to its original value anyway.

    To build up CO2 in the water and lower the pH permanently, you don't have to add additional CO2, just reduce the aeration and the pH will fall. However, photosynthesis by plants or algae will remove CO2 in the daytime so the pH would yo-yo up and down between its night time and daytime values.

    To have a lower stable pH, you have to use RO which is expensive to buy but easy to set up and produce a pH at your chosen value. Alternatively, you can have permanent acid injection controlled by pH measuring equipment but these are not "set and forget" systems. If they aren't maintained and calibrated regularly, they can go completely haywire and either not dose anything or continuously dose acid even if that causes a pH crash.

  14. #9
    Reading the above seems to suggest that having oyster shells is in reality a waste of time and effort. I think as thinking of removing mine anyway as I wondered if they were potentially going to collect gunk rather than anything else bc and this seems to support that idea

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